As in U.S. patents, such as Muller U.S. Pat. No. 3,434,427, Parks U.S. Pat. No. 2,765,868, Bouskill U.S. Pat. No. 3,396,825, Morrison U.S. Pat. No. 3,447,684 and Akerman U.S. Pat. No. 3,395,511, it is common practice for maintaining an uninterrupted output of filtered fluid to have a pair of filter units, one of which is being regenerated while the other is filtering and vice versa. When, as in Muller, Bouskill and Morrison, the fluid is a liquid, it also is not uncommon to connect the filtered liquid outlets of the two units and utilize the filtered liquid from the filtering unit for reverse-flow regeneration of the filter medium in the other unit. Parks even applies this latter practice to the filtering of compressed gas, but only as a supplement to the heating of the desiccant upon which he mainly depends for regenerating. Contrary to current practice as disclosed in Glass U.S. Pat. No. 3,060,393 and Hankison U.S. Pat. No. 3,464,186, Parks did not appreciate that in filtering a compressed gas with a desiccant, the efficiency of the regeneration is enhanced by expanding the regenerating gas before it reaches the desiccant and conducting the regeneration at a pressure approaching ambient.
Initially, as in Kennedy U.S. Pat. No. 2,955,673, the filtered gas used for the low pressure regeneration was drawn from the main reservoir of the compressed gas system in which the filter assembly was installed. However, to comply with federal and state regulations prohibiting diversion of main reservoir air in systems used for operating vehicular air brakes, the current practice is to store part of the output of filtered gas in a so-called purge or regenerating tank separate from the main reservoir and later draw the regenerating gas from that tank. This current practice, as applied to a continuous output dual filter assembly for compressed gas, is exemplified by Akerman's use of a purge tank separate from the main reservoir for each filter unit.
The shortcoming of separate purge tanks, such as disclosed in Akerman, is that, as the regenerating proceeds, the pressure in the purge tank is progressively reduced, with consequent reduction in the flow and regenerating efficiency of the low pressure gas passing through the desiccant. This shortcoming appears to have been first recognized in my copending application Ser. No. 766,437, filed Feb. 7, 1977, in which by using a common purge tank and the substantially constant pressure of the gas in the filtering unit, a constant reverse flow of low pressure gas through the desiccant in the other unit is maintained throughout the regeneration. It is to improvements on the assembly of my copending application that the present invention is particularly directed.